Direct reading sound pressure meter



Feb. 19, 1946. F, M. WIENER E1- AL 2,395,309

-' DIRECT READING sounn PRESSURE METER Filed Sept. so, 1943 FRANCIS N. W/ENER v ROBERTLh/ALLACE (JR. INVENTORS B 1J JJ I ATTORNEY Patented Feb. 19, 1 946 UNITED *STATE s v PAT NT OFFICE DIRECT READING SOUND PRESSURE ETER Francis M. Wiener and Cambridge, Mass., of America, the Navy Robert L. Wallace, Jr., assignors to United- States as represented by the Secretary of Application September 30, 1943, Serial No. 504,488 Y will produce a voltage at its output proportional to' the sound pressure and of such magnitude that it can be read on a high impedance voltmeter. The output impedance of the device is low enough so as not to introduce any appreciable error due to the finite impedance of the voltmeter.

A more specific object is to provide a device of the character described, utilizing a high impedance condenser type microphone as the sound responsive element and in which a novel circuit arrangement is utilized to provide a high impedance input to an amplifier tofwhich the microphone is connected, and which also furnishes a suitable biasing potential for the microphone to measure its open circuit voltage, which is propor-' tional to the sound pressure.

These and other objects of our invention will become apparent from the detailed description to follow and from the accompanying drawing, the single figure of which shows schematically a preferred embodiment of our invention. 7

Referring now to the drawing, the numeral I designates an amplifier tube which may be, for example, an RCA type 6J7 or 1620. Tube I0 is connected as a triode cathode follower and includes a plate suppressor grid l2, screen grid l3, control grid l4, cathode l and a cathode heater l0. p

A condenser type microphone is shown at H, and is preferably a Western Electric type' 640-A. It has one plate I9 connected to control grid I l of tube Hi. The other plate l8 which is the movable member of the microphone is connected to the lower end of a plurality of serially arranged resistors 20, 2| and 22. The upper end of resistor 22 is connected to cathode IS. The positive sideof a 300 volt "B" supply source isfconnected tov plate ll of tube |0 and the negative side thereof is connected to the lower end of resistor 'and round 30.

The values of resistors 20, 2| and 22 are so chosen that a '200vo1t biasing potential for the microphone I! will be obtained across resistor 20 and the tapped portion 01' the resistor 2|. The remainder of resistors 2| and 22 furnish approximately 2 volts for the bias on tube l0. Satisfac-- tory values for these resistors which are prefer- ,ably wire wound are as follows: Resistor 20-0.1

3 Claims. (Cl. 181-05) megohm, resistor 2l-400 ohms and resistor 22- 1000 ohms.

As the biasing potential across microphone I! should remain constant in order for the sound pressure reading to be consistently accurate,'we.

preferably include a voltmeter 24 which is connectedacross the tapped portion of resistor 2| and resistor 20. The potentiometer arrangement of resistor 2| is so adjusted as to vary current flow through the resistors until voltmeter 24 reads 200 volts, which thus establishes the desired 200 volt bias on microphone II. A proper tube bias of about -2 volts is provided for tube l0 so that no grid current flows.

The bias circuit for the microphone I I also includes a 20 megohm resistor 23. It prevents the charge generated bythe motion of the movable plate l8 of .the condenser microphone from leaking ofi. It is very important for the proper functioning of the circuit for other reasons also, which will be discussed later.

Also connected across resistor 2| to 22 inclusive, is a 2 me'gohm resistor 25 and a .5 microfarad condenser 26. The condenser 26 keeps the direct current potential of about 200 v. between cathode and ground from appearing across resistor 25, and vacuum tube voltmeter 29, whereas its impedance for alternating currents is very small so that effectively the full output voltage ez appears across resistor 25. A rheostat 21 is in.- cluded in the cathode heater circuit to regulate current flow therein, and another voltmeter 28 may be also connected in this circuit across the cathode heater l0 if desired. A high impedance electronic voltmeter 29 such'as the Ballantine electronic voltmeter model 300 is connected as shown. .This type of voltmeter produces at its output about 1 millivolt for a soun pressure level of decibels (re .0002 dynes/cm?) and its scale may be calibrated to read' directly in decibels sound pressure.

The operation of the device is as follows: When the microphone I1 is placed in a sound field; the alternating sound pressure of which it is desired to measure, a periodic motion of plate is is produced which generates the open circuit voltage of the microphone. This-open circuit voltage is applied across the high impedance input of tube In and associated circuit, and produces an output voltage e; across the resistor combination 20, 2| and 22, which essentiallyv appears in the scale reading of the high impedance voltmeter 29 which as heretofore indicated, may be calibrated to read directly in units of sound pressure.

That the voltage measured by voltmeter 29 is substantially equal to the voltage caused by the change in distance between plates l8 and IQ of microphone I1 is shown to be so by the following mathematical analysis:

Let er, ez, 65, i1, and i represent small signal variational voltages and currents respectively.

6z+8 =1 8g=Ri +R2i Where R E R1 But R e e R1,= R 1; is) Where =amp1ification factor r,,=plate resistance tube 10 From (1) and (3) e..[1+,' 'f]=e. (4)

And

T '1-" (t) (m) Where s =plate transconductance of tube Hi In the circuit shown in the drawing R 100,000 ohms 0 Sp 1700 x amperes/volt Hence independent of frequency.

To measure the open circuit voltage of a high impedance device, such as the condenser type microphone I'H whose impedance at 100 C. P. S. is about 3x 0 reactive ohms, the input impedance of the measuring device must be a multiple of that value.

That this is accomplished by this device is shown by the following mathematical analysis:

Since And the input impedance Zm from grid H to ground 30 becomes Now, in the circuit shown R,=2X 10 ohms and z,,=3.3 1o resistive ohms.

' the circuit constants given, namely of the pedance is proportional to Re, the frequency range can be extended towards lower frequencies by increasing Rg, i. e. resistor 23.

By similar calculations, it can be shown that the output impedance-of the device, measured between cathode l5 and ground 30 is very low for order of 1000 ohms or less. a

Summary (1) The device measures the open circuit voltage of a condenser microphone I1, which is proportional to the sound pressure acting upon it by means of a high impedance voltmeter 29, substantially independent of frequency over a wide audio frequency range. The voltmeter can be calibrated directly in terms of sound pressure.

(2) The performance of the device is substantially independent of changes in the characteristics of the vacuum tube 10.

(3) The output impedance of the device is sufficiently low to permit the reading of the voltmeter 29 at a location distant from the point of location of the microphone and amplifier without elaborate precautions and errors due to the finite impedance of the voltmeter.

(4) In a preferred embodiment of the invention, the microphone and amplifier are housed in a case of suitably small dimensions so as not to disturb appreciably the sound field to be measured.

(5) An insert resistor for precision measurement of the open circuit voltage generated by the microphone can be easily installed by breaking the lead between point 88 in the diagram and ground 3%. I

(6) Despite the fact that a. very high input impedance for alternating signals is realized, the resistance for direct current from the fixed plate i9 of the condenser microphone I! has the magnitude of R; which, from (7 is seen to be smaller than said input impedance, by a factor of approximately 16. Thus the eiiect of leakage in the condenser microphone is generally reduced compared with devices of similar kind known to the art prior to this invention. Moreover, the presence of leakage in the microphone reducing the value of the cflective bias voltage across the condenser microphone from the existing voltage across resistor R2 is indicated by a change in deflection of voltmeter 24 when the microphone is alternately connected and disconnected from the circuit.

(7) To make the deflection of voltmeter 29 independent of frequency for microphones whose open circuit voltage is not independent of frequency for a given sound pressure, an equalizing network can be inserted into the circuit between the output terminals of the device'and voltmeter 29, compensating for the response-frequency characteristic of the microphone.

Having thusfully described our invention, we claim:

1. In apparatus for measuring the intensity of a sound field, a space discharge tube having a cathode, control grid and plate, a source of unidirectional potential, an impedance, means connecting said source of potential in circuit with said plate and cathode through said impedance. a high impedance condenser microphone adapted to be placed in said sound field, a high impedance connecting said microphone into the plate-cathode circuit of said tube whereby the alternating potential produced across the condenser plates of said microphone when the latter is placed in said 2: Thecombi'nation in claim 1 and further including means utilizing the drop inpotentlal.

across at-least a portion of first said impedance for impressing a unidirectional biasing potential on the condenser plates oi said microphone.

3. In apparatus for .measuringthe intensity of a sound field a space discharge tube having a cathode, control grid and plate,-a source 01' unidirectional potential. an impedance, means connecting'said source of potential in circuit with said plate and cathode through said impedance, a high impedance condenser microphone adapted said p tential.

phone to the grid of said tube and also to-one point on first said impedance through a second and high impedance, circuit means connecting the other condenser plate of said microphone to another point on first said impedance whereby a biasing potential for the microphone is su'imlied" and whereby the alternating potential produced across the plates of said microphone when placed in said sound fleld will produce a substantially equivalent alternating potential across first said impedance. and means adapted to measurelast mmcrsmwmnm n nwarmcnaa.

to be placed in said sound field, circuit means connecting one condenser plate oi. said micro- 

